Heavy tires with a relatively thick tread region cross-section typically experience a significant heat build up and a corresponding increase in operating temperature as the tire is being worked on an associated vehicle.
Such heavy tires may be, for example, various large off-the-road tires as well as various truck tires, as compared to significantly smaller tires intended for use on passenger automobiles. Heavy tires for consideration herein may have tread region cross-section of a thickness of two inches (5 centimeters) or more. Heat build up in a tire tread during the working of such heavy tires under load can be rapid to thereby cause such tire treads to operate at relatively high temperatures. For such heavy tires with relatively thick tread region cross-section, heat durability and wear resistance of the tire tread is a factor for the longevity of the tire itself.
Accordingly, it is desired to provide such heavy tires having relatively thick tread region cross-section with one or more components, particularly a tread, where the component, namely a tread, can serve as a thermally conductive path for dissipation of heat from the tire. While the tire tread is a principal consideration for such thermally conductive path, other tire components for such purpose include, for example, tire sidewalls, internal tire shoulder wedges, tire sidewall inserts and tire apexes (apexes in the sidewall adjacent to a tire bead). Large tires, such as for example off-the-road tires, may sometimes contain internal blocks of rubber in their shoulder region in the vicinity of the region where the tire sidewall portion joins the tread portion and such tire components are referred to herein as tire shoulder wedges. Significant heat build-up can occur in this region during the working of the tire so that a thermally conductive path involving such shoulder wedge is desirable. Virtually all large tires contain internal sidewall inserts adjacent to their beads which extend radially outward into the tire sidewall which are referred as tire sidewall apexes which add stiffness and stability to the tire sidewall. Significant heat build-up can occur in this region during the working of the tire, particularly for aircraft tires which experience heavy loads during airplane take-offs and landings, so that a thermally conductive path involving such apex is desirable. Some tires may contain additional internal sidewall rubber inserts in addition to a tire apex to provide additional stiffness and stability for a tire sidewall, particularly where the tire may be desired to operate under reduced internal air pressure, (e.g. where the tire goes flat) and such additional sidewall rubber inserts are simply referred to therein as tire sidewall rubber inserts. To reduce significant heat build-up for such sidewall inserts a thermally conductive path involving such sidewall rubber inserts is desirable.
In practice, it is well known that thermal conductivity of a rubber composition can typically be improved by simply increasing its reinforcing carbon black content. However, it is also well known that increasing the reinforcing carbon black content of a rubber composition typically makes it more hysteretic in nature and therefore more prone to excessive generation of heat as it is being worked. These two phenomena oppose each other for such a tire component.
Additionally, heavy tire treads may contain, if desired, significant amounts of silica reinforcement in place of a significant amount of carbon black. In general, the increase in the silica content of the tread, primarily because of the reduction in the carbon black content of the tread rubber composition, tends to thereby reduce the tread's thermal conductivity and consequently increase the tread's heat generation when in service under working conditions.
Thin strips of electrically conductive rubber compositions which contain significant amounts of rubber reinforcing carbon blacks have been used to provide an electrically conductive path for dissipation of static electricity through or around silica reinforced tire treads which have limited electrical conductivity because of their relatively low contents of carbon black reinforcement. Such strips therefore do not constitute the entire tire tread and are typically necessarily thin in nature in order to not disturb various physical properties desired for the tire tread itself, such as for example one or more of the tire's rolling resistance, wet traction and treadwear.
For this invention, however, it is desired to provide a significantly more substantial path for thermal conductivity for a heavy tire tread by maximizing thermal conductivity for an entire rubber component, particularly the tire tread. Indeed, insofar as providing thermal conductivity for dissipation of heat, it is readily recognized that a thin rubber strip of a thermally conductive rubber composition through, over or around a tire tread for such purpose would be impracticable for such purpose.
In practice, it is also known to use highly conductive carbon blacks, such as for example acetylene-derived carbon blacks, in rubber compositions used for tire cure bladders to provide a shorter cure time for tires where it is desired to provide a thermally conductive path for greater heat transfer through the tire curing bladder to the tire itself to shorten the respective tire cure cycle. However, such acetylene-derived carbon blacks are not normally used in rubber compositions for tire components and particularly not normally used for treads for heavy tires.
Reinforcement of various rubber compositions for various tire components has been heretofore suggested. For example, see U.S. Pat. Nos. 5,430,087 and 5,798,405. However, it is considered herein that use of a combination of diverse carbon blacks provided herein for heavy tire components, particularly tire treads, for providing a path of thermal conductivity is of a significant departure from past practice.
In the description of this invention, the term “phr,” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer,” if used herein, may be used interchangeably, unless otherwise prescribed. The terms “rubber composition,” “compounded rubber” and “rubber compound,” if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the DBP absorption values for carbon blacks is a dibutylphthalate value expressed in terms of cm3/100 grams according to ASTM D2414. The Nitrogen Surface Area (referred to herein as “NSA”) value is expressed in terms of square meters per gram (m2/g) according to ASTM D3037.